Many applications of hydraulic fluids require that the viscosity of the fluid change as little as possible over the intended operating temperature range. Silicone fluids, and polydimethylsiloxane fluids in particular, are known to have a low viscosity variation as a function of temperature relative to organic oils. This property, along with its thermal and oxidative stability, often makes polydimethylsiloxane oil the hydraulic fluid of choice in many high performance applications.
In some critical applications, however, the ideal hydraulic fluid would show essentially no change in viscosity over the temperature range of interest. Thus, material scientists continue to search for a practical system which would approach this ideal. One traditional method used to reduce the variation of viscosity with temperature of petroleum oils (e.g., automotive motor oil) entails the dissolution of a high molecular weight polymer (e.g., polymethylmethacrylate) in the oil. Unfortunately, this approach has not been found to be very effective in the case of polydimethylsiloxane oils. Thus, the viscosity temperature coefficient (VTC), defined herein by the expression EQU VTC=1-(viscosity @100.degree. C./viscosity @38.degree. C.)
remains essentially unchanged at about 0.6 when a typical polydimethylsiloxane oil, having a viscosity of about 100 to 10,000 cP, is modified with several weight percent of a high molecular weight polydimethylsiloxane gum. Naturally, in the aforementioned applications, as low a VTC value as possible is preferred.